Optical wavelength shifting

ABSTRACT

An apparatus ( 8, 14 ) for wavelength shifting light from a first frequency ν 1  to a second frequency ν 1  is provided in the form of a semiconductor intersubband laser ( 8 ) lasing at its intersubband frequency ν 3 . The output light is wavelength shifted to a frequency ν 2 =ν 1 +−nν 3 , where n is a non-zero integer ( . . . − −2, −1, 1, 2  . . . ). The wavelength shifted light ν 2  may be amplitude modulated and/or frequency modulated to impart information upon it. The wavelength shifting is a coherent process allowing for the possibility of coherent communication techniques to be used.

This invention relates to the field of optical devices. Moreparticularly, this invention relates to an apparatus and method forchanging the wavelength of light in a manner that can, for example, beuseful in optical fiber systems.

It is known to provide semiconductor diode lasers as sources of laserlight that are robust and inexpensive. Such semiconductor diode laserstypically operate using a transition between the valence and conductionbands within the semiconductor material. It is also known to providequantum well semiconductor diode lasers in which the semiconductormaterial composition is carefully varied to yield a desired band gap ina manner that can tune the wavelength of the laser light produced.

More recently a class of semiconductor lasers have been developed thatare termed quantum cascade lasers. An example of such a laser isdescribed in the paper “Long wavelength infrared (λ=11 μm) quantumcascade lasers”, C. Sirtori, et al. Applied Physics Letters 69 (19), 4Nov. 1996, page 2810. These quantum cascade lasers operate using anintersubband transition within a quantum well structure. The device isarranged such that an electron undergoing a lasing transition in onequantum well can tunnel its way to an adjacent quantum well where itwill be energetically at the correct level to undergo a further lasingtransition. This cascade behavior allows the efficiency of the laser tobe improved.

It is known from the paper “Generation of first-order terahertz opticalsidebands in asymmetric coupled quantum wells”, C Phillips et al,Applied Physics Letters 75(18), 1 Nov. 1999, page 2728 that a quantumwell structure may be illuminated with light of two differentwavelengths and sidebands induced spaced around one of the incidentwavelengths. The system used in this experiment employed a Ti:Sapphirelaser together with a free electron laser as the light sources. The sizeand complexity of these laser sources precludes their use as a practicalway to shift the wavelength of light from an incident wavelength into asideband wavelength.

There is a constant need to increase the data transmission capabilitiesof communication systems. The adoption of optical fiber communicationsystems has markedly increased available bandwidth. A problem in suchoptical fiber communication systems is the need to avoid changing asignal from an optical signal into an electrical signal more than isabsolutely necessary. Electrical signal processing systems that arecapable of keeping pace with an optical fiber communication systems aredifficult to produce and expensive as well as representing a bottleneckin the transmission capabilities of the system.

A desired manipulation upon optical signals within an optical fibercommunication system is wavelength shifting. Such wavelength shiftingfacilitates wavelength division multiplexing that can release morebandwidth from a given optical fiber link. However, wavelength shiftingby receiving an optical signal and converting it into an electricalsignal that triggers the production of a further optical signal at adifferent wavelength suffers from the disadvantage of having to changethe signal from an optical form into an electrical form and then backinto an optical form as discussed above.

Viewed from one aspect the present invention provides apparatus fortransforming electromagnetic radiation at a first frequency ν₁ toelectromagnetic radiation at a second frequency ν₂, said apparatuscomprising:

-   -   a semiconductor intersubband laser operable to lase to generate        electromagnetic radiation at a third frequency ν₃; and    -   a radiation guide operable to direct electromagnetic radiation        at said first frequency ν₁ into said semiconductor intersubband        laser;    -   whereby, in operation, said electromagnetic radiation at said        first frequency ν₁ and said electromagnetic radiation at said        third frequency ν₃ undergo coherent frequency mixing within said        semiconductor intersubband laser to generate said        electromagnetic radiation of said second frequency ν₂, ν₂ being        ν₁+nν₃ with n being a non zero integer.

The present invention recognizes that a semiconductor intersubband laseris able to provide the electron energy level structure required toobtain wavelength shifting together with one of the electromagneticradiation fields in the form of the laser light of that laser itselfAccordingly, the additional components needed for wavelength shiftingare substantially only the incident electromagnetic radiation of thefirst frequency that can be passed into the semiconductor intersubbandlaser. This makes wavelength shifting a practical and economicalpossibility using what may be only a simple two-terminal device in theform of a semiconducter intersubband laser. The wavelength shiftingoccurs as an optical process without the need to convert into anelectrical signal thereby avoiding the processing bottlenecks associatedwith conversions into electrical signals. The frequency mixing thatoccurs within the semiconducting intersubband laser is a coherentprocess and accordingly lends itself to coherent communication schemes.The wavelength shift induced (“channel separation”) can be tuned byvarying the intersubband gap using known quantum well techniques to suitthe particular requirements. The wavelength can be both increased anddecreased as n can be both positive and negative. It is also possible tomodulate the sidebands using amplitude modulation or frequencymodulation as an additional way of imparting information onto theoptical signals.

It will be appreciated that the semiconductor intersubband laser couldhave various different forms. However, a quantum cascade laser isparticularly well suited for this use in view of its high efficiency andthe ability to engineer its photon energy.

A problem that reduces the intensity of the sideband radiation is itthis may be absorbed within the semiconductor intersubband laser. Theamount of wavelength shifted electromagnetic radiation emerging from theintersubband laser can be improved by using a mirror to reflectelectromagnetic radiation back out of the semiconductor intersubbandlaser.

The mirror preferably abuts a face of the semiconductor intersubbandlaser and may have the form of a multilayer Bragg reflector upon whichthe semiconductor intersubband laser is formed (grown).

The radiation guide for directing the electromagnetic radiation of thefirst frequency into the semiconductor intersubband laser could takevarious different forms. A particularly well suited form is an opticalfiber, although a suitable lens and free transmission arrangement wouldbe possible.

If an optical fiber is used as the radiation guide, then this may beconveniently butt coupled to the semiconductor intersubband laser andused to collect the electromagnetic radiation at the second frequency aswell as inject the electromagnetic radiation at the first frequency.

As previously mentioned, modulation may be applied to the semiconductorintersubband laser to modulate the wavelength shifted light both inamplitude (including simply on and off) and in frequency. The currentflow through the semiconductor intersubband laser alters its refractiveindex which in turn alters the wavelength of the electromagneticradiation of the third frequency and consequently also the wavelength ofthe electromagnetic radiation at the second frequency.

In the context of frequency modulation, it is desirable that thesemiconductor intersubband laser should include a distributed feedbackgrating to constrain the third frequency to avoid this becomingunstable.

Whilst the system may operate over a considerable range of frequencies,preferred frequencies are ones in which the electromagnetic radiation ofthe first is frequency is near infrared radiation and theelectromagnetic radiation of the third frequency is infrared radiation.

The efficiency of the wavelength shifting is strongly enhanced when theelectromagnetic radiation of the first frequency and the electromagneticradiation of the second frequency are both substantially resonant withelectron transitions within the semiconductor intersubband laser.

As previously mentioned, the present invention is particularly useful inproviding a multiplexer for use in wavelength division multiplexingsystems or an amplitude or frequency modulator for optical signals.

Viewed from another aspect the invention provides a method oftransforming electromagnetic radiation at a first frequency v, toelectromagnetic radiation at a second frequency ν₂, said methodcomprising the steps of:

-   -   generating electromagnetic radiation at a third frequency ν₃        with a semiconductor intersubband laser; and    -   directing electromagnetic radiation at said first frequency ν₁        into said semiconductor intersubband laser;    -   whereby, in operation, said electromagnetic radiation at said        first frequency ν₁ and said electromagnetic radiation at said        third frequency ν₃ undergo coherent frequency mixing within said        semiconductor intersubband laser to generate said        electromagnetic radiation of said second frequency ν₂, ν₂ being        ν₁+nν₃ with n being a non zero integer.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIG. 1 schematically illustrates the electron energy levels andtransitions within a semiconductor intersubband laser;

FIG. 2 schematically illustrates the electron energy levels andtransitions within a quantum cascade laser;

FIG. 3 illustrates a semiconductor intersubband laser operating as awavelength shifting device;

FIG. 4 illustrates three alternative ways in which light may be coupledin and out of a semiconductor intersubband laser;

FIG. 5 illustrates input and output spectra;

FIG. 6 illustrates modulation applied to the system;

FIG. 7 illustrates the effects of modulation; and

FIG. 8 schematically illustrates a wavelength division multiplexedoptical communication system.

FIG. 1 shows the energy level structure within a semiconductorintersubband laser. Intersubband lasing transitions occur between theenergy levels 2 and 4. These may typically yield output light of awavelength of 10 μm. The energy difference between the energy levels 2and 4 is E_(III) which corresponds to photon frequencies of a thirdfrequency ν₃. In operation, when the semiconductor intersubband laser iselectrically driven lasing occurs due to an electron transition from theenergy level 2 to the energy level 4. In the normal way, reflective endfaces of the laser result in electromagnetic radiation being trappedwithin the laser body for multiple passes and yielding a high intensityof electromagnetic radiation at the third frequency ν₃.

The semiconductor intersubband laser also provides an electron energytransition from an energy level 6 within the valance band to the energylevel 4 within the conduction band. This transition, of energy E_(I),may typically be about ten times higher in energy than the intersubbandtransition and accordingly may have an associated wavelength ofapproximately 1 μm. This energy transition is not used during the normaloperation of an intersubband laser. However, the present invention usesthis transition to provide frequency mixing between inputelectromagnetic radiation of a first frequency tuned to this transitionwith an energy E_(I) and the high intensity electromagnetic radiation ofenergy E_(III) due to the lasing action described above. It will be seenthat the semiconductor intersubband laser provides electron transitionsresonant with both the incident electromagnetic radiation at an energyE_(I) and the lasing electromagnetic radiation at an energy E_(III). Theresult of the interaction within the semiconductor intersubband laser ofthese two frequencies of electromagnetic radiation and the electronenergy level structure couples some energy into electromagneticradiation sidebands at a second frequency ν₂ corresponding to an energyEl,. This electromagnetic radiation may be at a variety of differentsideband frequencies represented by E_(II)=E_(I)+nE_(III), where n=. . .−2,−1,1,2 . . . The electromagnetic radiation at the second frequency ν₂and with the energy E_(II) is wavelength shifted from the incidentelectromagnetic radiation at the first frequency ν and energy E_(I).This wavelength shift is highly desirable and difficult to achieve inother ways. In particular, converting the input light into an electricalsignal and then generating a new optical signal at a new wavelength isan alternative, but significantly less attractive, proposition. Theoutput electromagnetic radiation with an energy E_(II) is coherent withthe input light giving the possibility for coherent communicationsystems and modulation techniques.

It will be appreciated that the efficiency with which light is movedinto the sidebands will vary depending upon the particular circumstancesand the degree of non-linear interaction that occurs. Whilst theabsolute intensity of the wavelength shifted light may not be high, itis a comparatively simple matter to optically amplify this wavelengthshifted light to a desired intensity. The number of sidebands producedwill similarly vary depending upon the particular circumstances and wayin which the system is driven. An individual sideband may be what isdesired for a particular wavelength shift and this may be selected usingappropriate filters. Alternatively, some modulation techniques mayutilize all of the generated sidebands.

FIG. 2 illustrates a quantum cascade laser electron energy levelstructure. This quantum cascade laser can be seen to be a form ofsemiconductor intersubband laser in that it provides within each quantumwell the structure illustrated in FIG. 1. However, when a givenintersubband transition has occurred within the quantum cascade laser,the electron may tunnel into the adjacent well where a furthertransition may occur. This significantly increases the efficiency of thelaser.

FIG. 3 illustrates the physical appearance of a semiconductorintersubband laser 8. This semiconductor intersubband laser 8 is formedupon a Bragg mirror 10 comprising alternating layers of differingrefractive index. A distributed feedback grating 12 is provided at oneend of the laser 8 to lock the laser to a specific range of lasingfrequencies.

In operation, the laser 8 is driven to lase at the third frequency ν₃.Input light at the first frequency₁ is directed into the laser 8 whereit interacts with the lasing light at the third frequency ν₃ to yieldwavelength shifted light ν₂ within sidebands of the incident light ofthe first frequency ν₁. This wavelength shifted light ν₂ is at one ormore different frequencies that are integer multiple differences awayfrom the input frequency ν₁.

FIG. 4 illustrates three alternative ways in which the input light maybe passed into the laser 8 and recovered from the laser 8. In theexample A, an optical fiber 14 is butt coupled to the laser 8 and passeslight into the laser 8 as well as collecting the wavelength shiftedlight from the laser 8 after it has been reflected from the Bragg mirror8. In the example B, an optical fiber 16 passes the input light into thelaser 8 and wavelength shifted light (as well as a considerable amountof the incident light) is collected out of the laser 8 by a secondoptical fiber 18. Example C illustrates input light being passed by freetransmission into the laser 8 and collected on the opposite side. Thisuses a combination of optical fibers and lenses. It will be appreciatedthat other light injection and collection geometrys would be possible.

FIG. 5 schematically illustrates the input and output spectra from thelaser 8. The input light is the electromagnetic radiation of the firstfrequency ν₁. The output light includes a dominant component at theinput frequency ν₁ together with various sideband components atwavelength shifted frequencies ν₂. These wavelength shifted frequenciesare spaced by integer multiples of the lasing frequency ν₃ of the laser8 away from the input frequency ν₁. The distribution profile of theintensity of the sidebands will vary depending upon the particulardriving conditions and other properties of the system.

FIG. 6 illustrates how the technique of the invention may be used toprovide modulation of an optical signal. Whilst the laser 8 may be asimple two-terminal device connected to a current source 20, the currentpassed through the laser 8 may be modulated to provide modulationcontrol. The modulation may simply switch on and off the lasing withinthe laser 8 in a manner that switches on and off the light within thesidebands. Alternatively, the light within the sidebands may bemodulated in intensity between non-zero values by controlling thecurrent through the laser to control the lasing light intensity betweennon-zero levels.

A further possibility is the use of frequency modulation of the sidebandlight. When the current through the laser 8 changes, the refractiveindex of the laser 8 changes. This change in refractive indexeffectively changes the cavity length of the laser 8 and so alters thewavelength of the laser light ν₃. Changing ν₃ also changes thefrequencies of the sidebands. Accordingly, modulating the currentthrough the laser 8 can impart a frequency modulation upon the sidebandlight ν₂. In this circumstance a distributed feedback grating would notbe used to lock the frequency ν₃.

FIG. 7 illustrates the different types of modulation that can be appliedto the sideband light at frequency ν₂. Amplitude modulation, such assimple on/off modulation or amplitude intensity modulation betweennon-zero values may be provided. Alternatively, the frequency of thesideband light ν₂ may be altered as described above to yield frequencymodulation.

FIG. 8 schematically illustrates a simple optical communications systemthat may employ wavelength division multiplexing. The example uses along distance portion 22 that is of a high capacity and uses light closeto a frequency VLD (e.g. 1.55 μm) best suited for long distancepropagation (e.g. low absorption and dispersion). As this long distanceportion may be difficult and expensive to duplicate (e.g. an underseacable), then one way of yielding more data bandwidth from it is to usewavelength division multiplexing into N channels around the frequencyν_(LD). The devices of the present invention may be used in thewavelength shifting needed to produce the wavelength divisonmultiplexing in the long distance portion.

In a local area, less capacity may be required and a more significantissue may be reducing the cost of the local system. Generally, equipmentfor transmitting and manipulating light signals is less expensive forlight of a shorter wavelength (e.g. 0.8 μm or 1.3 μm). The light withinthe long distance portion may be separated into different channels usingnarrow line filters and then the devices of the present invention usedto wavelength shift these separated signals to wavelengths more suitedto the local transmission requirements. Whilst this wavelength shiftingcould be achieved by transforming the long distances light pulses intoelectrical signals and then using these electrical signals to generateoptical pulses at the different wavelength, this process isfundamentally less attractive (e.g. inexpensive, flexible, . . . ) thanone that takes place purely in the optical domain. Accordingly, thepresent invention provides the capability for wavelength shifting theoptical pulses from a wavelength suited for long distance transmissionto one wavelengths used for local transmission. The multiplexer can bethought of as serving to receive the input pulses at the inputwavelength and then directing them into a different wavelength channelas optical pulses at that different wavelength. The wavelength shiftedpulses may require amplification prior to transmission along their localpath, but amplification may be relatively readily provided in theoptical domain.

1. Apparatus for transforming electromagnetic radiation at a firstfrequency ν₁ to electromagnetic radiation at a second frequency ν₂, saidapparatus comprising: a semiconductor intersubband laser operable tolase to generate electromagnetic radiation at a third frequency ν₃; anda radiation guide operable to direct electromagnetic radiation at saidfirst frequency ν₁ into said semiconductor intersubband laser; whereby,in operation, said electromagnetic radiation at said first frequency v,and said electromagnetic radiation at said third frequency ν₃ undergocoherent frequency mixing within said semiconductor intersubband laserto generate said electromagnetic radiation of said second frequency ν₂,ν₂ being ν₁+nν₃ with n being a non zero integer.
 2. Apparatus as claimedin claim 1, wherein said semiconductor intersubband laser is a quantumcascade laser.
 3. Apparatus as claimed in claim 1, further comprising amirror operable to reflect electromagnetic radiation directed into saidsemiconductor intersubband laser by said radiation guide to pass out ofsaid semiconductor intersubband laser.
 4. Apparatus as claimed in claim3, wherein said mirror abuts a face of said semiconductor intersubbandlaser.
 5. Apparatus as claimed in claim 3, wherein said mirror is adistributed Bragg reflector.
 6. Apparatus as claimed in claim 4, whereinsaid semiconductor intersubband laser is formed upon said mirror. 7.Apparatus as claimed in claim 1, wherein said radiation guide is anoptical fiber.
 8. Apparatus as claimed in claim 7, wherein said opticalfiber is butt coupled to said semiconductor intersubband laser. 9.Apparatus as claimed in claim 1, wherein said electromagnetic radiationat said second frequency ν₂ is output along said radiation guide. 10.Apparatus as claimed in claim 1, wherein said electromagnetic radiationat said first frequency ν₁ is transmitted through said semiconductorintersubband laser to emerge mixed with said electromagnetic radiationat said second frequency ν₂.
 11. Apparatus as claimed in claim 1,further comprising a laser controller coupled to said semiconductorintersubband laser and operable to amplitude modulate saidelectromagnetic radiation at said third frequency ν₃ so as to amplitudemodulate said electromagnetic radiation at said second frequency ν₂. 12.Apparatus as claimed in claim 11, wherein said laser controlleramplitude modulates said electromagnetic radiation at said thirdfrequency ν₃ by controlling current following through said semiconductorintersubband laser.
 13. (Currently Amended) Apparatus as claimed inclaim 1, further comprising a laser controller coupled to saidsemiconductor intersubband laser and operable to frequency modulate saidelectromagnetic radiation at said third frequency ν₃ so as to frequencymodulate said electromagnetic radiation at said second frequency ν₂. 14.Apparatus as claimed in claim 13, wherein said laser controllerfrequency modulates said electromagnetic radiation at said thirdfrequency ν₃ by controlling current following through said semiconductorintersubband laser to alter a refractive index of said semiconductorintersubband laser in a manner that alters said third frequency ν₃. 15.Apparatus as claimed in claim 1, wherein said semiconductor intersubbandlaser includes a distributed feedback grating to constrain said thirdfrequency ν₃.
 16. Apparatus as claimed in claim 1, wherein saidelectromagnetic radiation of said first frequency ν₁ is near infraredradiation.
 17. Apparatus as claimed in claim 1, wherein saidelectromagnetic radiation of said third frequency ν₃ is infraredradiation.
 18. Apparatus as claimed in claim 1, wherein saidelectromagnetic radiation at said first frequency ν₁ and saidelectromagnetic radiation at said second frequency ν₂ are bothsubstantially resonant with electron transitions within saidsemiconductor intersubband laser.
 19. A multiplexer for wavelengthdivision multiplexing signals propagating along an optical fiber, saidmultiplexer including apparatus as claimed in claim
 1. 20. An amplitudemodulator for amplitude modulating electromagnetic radiation, saidamplitude modulator including apparatus as claimed in claim
 1. 21. Afrequency modulator for frequency modulating electromagnetic radiation,said frequency modulator including apparatus as claimed in claim
 1. 22.A method of transforming electromagnetic radiation at a first frequencyv, to electromagnetic radiation at a second frequency ν₂, said methodcomprising the steps of: generating electromagnetic radiation at a thirdfrequency ν₃ with a semiconductor intersubband laser; and directingelectromagnetic radiation at said first frequency ν₁ into saidsemiconductor intersubband laser; whereby, in operation, saidelectromagnetic radiation at said first frequency ν₁ and saidelectromagnetic radiation at said third frequency ν₃ undergo coherentfrequency mixing within said semiconductor intersubband laser togenerate said electromagnetic radiation of said second frequency ν₂, ν₂being ν₁+nν₃ with n being a non zero integer.
 23. A method of wavelengthdivision multiplexing signals propagating along an optical fiber, saidmethod including the steps as claimed in claim
 22. 24. A method ofamplitude modulating electromagnetic radiation, said method includingthe steps as claimed in claim
 22. 25. A method of frequency modulatingelectromagnetic radiation, said method including the steps as claimed inclaim 22.